This invention relates generally to a high efficiency regulator for a power supply and more particularly to a reactive buck automatic D.C. voltage regulator for controlling the rectified D.C. output of a loosely coupled transformer thereby providing load regulation from an unregulated alternating current source at maximum efficiency.
At an early date, power supplies were designed which converted an A.C. input signal into a rectified D.C. output. The need for regulating D.C. output under varying load and input conditions was immediately apparent for many applications. Many early attempts at regulation employed the use of a saturable reactor transformer having a well-defined saturation characteristic wherein the half cyclic average voltage induced in one winding on the saturable reactor was constant so long as the core was saturated and the frequency was fixed. This was true regardless of the magnitude of the waveform of the input voltage and regardless of the number of turns on the primary winding since, so long as the core was driven into saturation, the output voltage follows the number of turns on the secondary winding. If a rectifier and averaging filter were added to follow the output of the saturable reactor transformer, the D.C. output was regulated for line voltage changes, and this quickly became the most fundamental type of line voltage regulator.
A more efficient line voltage regulator employed the use of an inductor in series with the saturable reactor transformer to reduce power losses, and still a more efficient technique for regulating the output voltage was developed which employed the use of a capacitor placed in parallel with the saturable reactor such that the capacitor and the series inductor could be tuned near the input frequency. This arrangement provides an almost unity power factor and efficient power transfer. These circuits were called ferroresonant regulators and enjoyed an improved power factor, a better output waveform for rectification and filtering, and relative insensitivity to input voltage spikes. Ferroresonant regulators, however, have five major disadvantages: (1) the output voltage is frequency sensitive; (2) since the core operates in saturation, the core losses are high and the external magnetic field is high; (3) since the output varies directly with the cross sectional area of the core, normal core tolerances cause unit-to-unit output voltage differences; (4) since the core is the regulating element, the output voltage varies with load current changes due to voltage drops in the secondary resistance; and (5) since the magnetic materials used in ferroresonant regulators have a inherent non-squareness to their characteristic B-H magnetic loops; ideal operation is impossible.
Feedback controlled ferroresonant voltage regulators were then designed in an attempt to overcome these disadvantages. These systems often employed a control circuit having an RC combination in which an integrater was used to measure the volt-time area of the output voltage. A series inductor was chosen to have a value approximately equal to the value of the saturated inductor in the secondary of the ferroresonant transformer and a triac switch was used which closes when there is sufficient gate current flowing into the device. The RC combination integrates the output voltage such that the peak voltage across the capacitor at any instant is proportional to the volt-time area of the voltage at the secondary of the transformer. When the voltage at the capacitor reaches a predetermined value, gate current flows to the triac switch which conducts current and causes the capacitor to discharge and recharge in the opposite direction through the series inductor. At this time the voltage across the switch is reversed causing it to come out of conduction thus completing a half cycle. This action occurs for each half cycle with the opposite polarity. Power losses are reduced since this action cannot occur if the core saturates since the necessary volt-time area required to fire the switch cannot be obtained. But this approach still wasted power since the energy which was shunted through the series inductor was lost; the output voltage was still frequency dependent, although not to the extent that existed in a conventional C.V.T.; and voltage drops in the secondary circuit in the rectifier bridge still affected the output voltage.
More sophisticated feedback controlled ferroresonant DC voltage regulators increase the accuracy of the regulation as a function of the design of the feedback circuit. In addition, the feedback control circuit allows the value of the series inductor to be made small which allows the switch to reverse more rapidly, producing a squarer wave and reducing the required filter capacitance. Since the transformer core does not saturate, core losses are reduced and the circuit is more efficient but some loss still occurs in the series inductor shunt circuit.
Other approaches to automatic voltage regulators have employed electromechanical means wherein a solid state detector is used to control a motor drive variable transformer and a buck-boost fixed-ratio transformer is used to vary the output voltage. When the output voltage deviates from its desired value, the detector produces an error signal which causes the variable transformer drive motor to actuate and drive the auxiliary buck-boost transformer so as to cause the voltage to return to the desired level. These systems suffer severe disadvantages in the speed of correction, the accuracy of the control, and the wear and breakdown of mechanical parts.